The Collaborative International Dictionary of English v.0.48:

Rod \Rod\, n. [The same word as rood. See Rood.]
   1. A straight and slender stick; a wand; hence, any slender
      bar, as of wood or metal (applied to various purposes).
      Specifically:
      (a) An instrument of punishment or correction;
          figuratively, chastisement.
          [1913 Webster]

                He that spareth his rod hateth his son. --Prov.
                                                  xiii. 24.
          [1913 Webster]
      (b) A kind of sceptor, or badge of office; hence,
          figuratively, power; authority; tyranny; oppression.
          "The rod, and bird of peace." --Shak.
      (c) A support for a fishing line; a fish pole. --Gay.
      (d) (Mach. & Structure) A member used in tension, as for
          sustaining a suspended weight, or in tension and
          compression, as for transmitting reciprocating motion,
          etc.; a connecting bar.
      (e) An instrument for measuring.
          [1913 Webster]

   2. A measure of length containing sixteen and a half feet; --
      called also perch, and pole.
      [1913 Webster]

   Black rod. See in the Vocabulary.

   Rods and cones (Anat.), the elongated cells or elements of
      the sensory layer of the retina, some of which are
      cylindrical, others somewhat conical.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rod
    n 1: a long thin implement made of metal or wood
    2: any rod-shaped bacterium
    3: a linear measure of 16.5 feet [syn: perch, rod, pole]
    4: a square rod of land [syn: perch, rod, pole]
    5: a visual receptor cell that is sensitive to dim light [syn:
       rod, rod cell, retinal rod]
    6: a gangster's pistol [syn: gat, rod]